Added docs about attestation token.

doc/CHANNELING-API.txt

@@ -68,7 +68,8 @@ Sending vs receiving document data encapsulation
         "From": "4",
         "To": "5",
         "Data": {},
-        "Iid": "request-identifier-unique-to-client"
+        "Iid": "request-identifier-unique-to-client",
+        "A": "attestation-session-token"
     }
 
   The Data key contains the real Document.
@@ -83,6 +84,7 @@ Sending vs receiving document data encapsulation
     Iid  : Optional request identifier to match this response to the calling
            request. Only available when sent by the client and the requested
            type implementation does support it.
+    A    : Session attestation token.
 
 
 Special purpose documents for channling
@@ -400,11 +402,16 @@ Information retrieval
         "Type": "Sessions",
         "Sessions": {
           "Type": "Token type",
-          "Token": "Request token"
+          "Token": "Token data"
         }
     }
 
-    Valid known token types are: "contact".
+    Valid known token types are:
+      contact:
+        Token data retrieved when a contact request is accepted.
+      session:
+        Token data retrieved on incoming messages as A field (attestation
+        token).
 
   Sessions (Response with Id, Token and Type from request and
   populated Session list).